The formation of an oil and gas well includes securing a metal casing within a wellbore via cement forming an annular structure between the sidewall of the wellbore and the outer diameter of the casing. Acoustic evaluation is often used to determine whether the cement provides hydraulic zonal isolation between formation strata traversed by the wellbore. Ultrasonic pulse-echo measurements and other acoustic measurements are used in cement evaluation, such as to provide the effective acoustic impedance of the annulus material adjacent to the casing and thereby evaluate cement characteristics and zonal isolation. For example, an ultrasonic pulse-echo tool may transmit a broadband pulse to the casing wall to excite a thickness resonance mode in the casing.